1. Field of the Invention
The present invention relates to a superconducting coil, and more particularly, to a concentric superconducting coil of a thin-film superconducting wire wound with a multilayer structure around a core into concentric shape so as to reduce a peel force in the superconducting coil with higher stability.
Each of the concentric superconducting coil has a non-circular shape including a racetrack shape, a saddle shape, an ellipse shape, an oval shape and a rectangle shape, and also has a circular shape.
2. Description of the Related Art
According to the progressing and advancing of superconducting technology, systems or apparatus for, for example, magnetic resonance imaging (MRI) diagnosis, superconducting magnetic energy storage (SMES), and crystal pulling have been practically used. In these systems or apparatus, superconducting tape wires of laminated members are wound into superconducting coils for an actual use, and generally, impregnated coils formed by resin impregnation are used in view of cooling and handling ease.
In an impregnated coil, however, a force is applied perpendicularly (in a peeling direction) to a longitudinal direction of a superconducting tape wire during cooling due to anisotropy of a coefficient of linear expansion of members.
Superconducting tape wires exhibit excellent mechanical properties, for instance, stress resistance, against a longitudinal force but are susceptible to a force applied in a peeling direction. Thus, impregnated coils of superconducting tape wires may unfortunately degrade superconducting characteristics during a cooling operation.
For this reason, in order to prevent distortion caused by a difference in coefficient of linear expansion between a core and a superconducting tape wire, there has been provided a method of winding wires without bonding an outer periphery of the core to an innermost turn of a coil such as disclosed in Japanese Patent Laid-Open Publication No. 2008-140905 (Patent Document 1).
Furthermore, in accordance with the tendency of the superconducting coil being larger, the diameter ratio (i.e., outside diameter/inside diameter) becomes also larger. Hence, a peel force generated in the coil increases. In the case where the peel force exceeds an allowable stress of a superconducting tape wire, superconducting characteristics may degrade.